Shaking caves. Just what is needed to make that happen?

[Roondar]

A staple of Sci-Fi shows and movies is that rooms, caves and ships start to shake when 'bad stuff' happens.

You know the scene - a small group of our heroes has taken refuge in a cave to prevent them from being hit by the enemies weapons. The enemy, not going to let them get away with it, starts bombing the area. The ground starts to shake, small bits (or less small) of the ceiling come down and the heroes make their stand... Waiting for the infantry dudes in silly costumes to arrive to attack/rescue them.

Now, what I'm wondering is: just how heavy would a bombardment need to be to actually give these results? What would make a room in a cave some ways underground shake visibly?

Is there an easy way to approximate the yield you'd need for a) a direct hit and b) a hit some ways off?

[Mr. Oragahn]

I don't know, I've seen it happen in Stargate Universe and I wondered what could shake the cave.

[Jedi Master Spock]

Yes. If you have a good idea of the magnitude of the shaking and the distance involved, you can estimate the order of magnitude of the yields involved.

We actually talked about this a little here, not too long after the forum was launched.